Capacitive micromachined ultrasonic transducers (cMUTs) are electrostatic actuator/transducers, which are widely used in various applications. Ultrasonic transducers can operate in a variety of media including liquids, solids and gas. These transducers are commonly used for medical imaging for diagnostics and therapy, biochemical imaging, non-destructive evaluation of materials, sonar, communication, proximity sensors, gas flow measurements, in-situ process monitoring, acoustic microscopy, underwater sensing and imaging, and many others. In addition to discrete ultrasound transducers, ultrasound transducer arrays containing multiple transducers have been also developed. For example, two-dimensional arrays of ultrasound transducers are developed for imaging applications.
Compared to the widely used piezoelectric (PZT) ultrasound transducer, the MUT has advantages in device fabrication method, bandwidth and operation temperature. For example, making arrays of conventional PZT transducers involves dicing and connecting individual piezoelectric elements. This process is fraught with difficulties and high expenses, not to mention the large input impedance mismatch problem presented by such elements to transmit/receiving electronics. In comparison, the micromachining techniques used in fabricating MUTs are much more capable in making such arrays. In terms of performance, the MUT demonstrates a dynamic performance comparable to that of PZT transducers. For these reasons, the MUT is becoming an attractive alternative to the piezoelectric (PZT) ultrasound transducers.
The basic structure of a cMUT is a parallel plate capacitor with a rigid bottom electrode and a top electrode residing on or within a flexible membrane, which is used to transmit (TX) or detect (RX) an acoustic wave in an adjacent medium. A DC bias voltage is applied between the electrodes to deflect the membrane to an optimum position for cMUT operation, usually with the goal of maximizing sensitivity and bandwidth. During transmission an AC signal is applied to the transducer. The alternating electrostatic force between the top electrode and the bottom electrode actuates the membrane in order to deliver acoustic energy into the medium surrounding the cMUT. During reception the impinging acoustic wave vibrates the membrane, thus altering the capacitance between the two electrodes. An electronic circuit detects this capacitance change.
Two representative types of cMUT structures are conventional flexible membrane cMUT and the newer embedded-spring cMUT (ESCMUT). FIG. 1 shows a schematic cross-sectional view of a conventional flexible membrane cMUT 10, which has a fixed substrate 101 having a bottom electrode 120, a flexible membrane 110 connected to the substrate 101 through membrane supports 130, and a movable top electrode 150. The flexible membrane 110 is spaced from the bottom electrode 120 by the membrane supports 130 to form a transducing space 160.
FIG. 2 is a schematic cross-sectional view of embedded-spring cMUT (ESCMUT) 200, which is described in the PCT International Application No. PCT/IB2006/051568, entitled MICRO-ELECTRO-MECHANICAL TRANSDUCERS, filed on May 18, 2006; and International Application (PCT) No. PCT/IB2006/051569, entitled MICRO-ELECTRO-MECHANICAL TRANSDUCERS, filed on May 18, 2006, particularly the cMUTs shown in FIGS. 5A-5D therein. The cMUT 200 has a substrate 201, on top of which is a spring anchor 203 supporting a spring layer 210; a surface plate 240 connected to the spring layer 210 through spring-plate connectors 230; and a top electrode 250 connected to the surface plate 240. The cMUT 200 may be only a portion of a complete cMUT element (not shown). Usually multiple cMUTs (cMUT elements) are used as an array device.
In addition to emitting into an intended medium, the acoustic energy may also leak into substrate. There are at least two different sources for this leakage. One is the acoustic energy from the moving components of the cMUT coupling into the substrate through anchors, and another is the electrostatic force directly applied on the substrate. The acoustic energy coupling into the substrate may introduce cross-talking between cMUT elements and an acoustic ring between the two surfaces of the cMUT substrate. There is therefore a need to minimize the effect of the acoustic energy leaking into the substrate.